This invention relates to a flow divider, and more particularly relates to a novel flow divider for equalizing the distribution of fluid fuel to the nozzles of a plurality of combustion cans of a gas turbine arrangement.
Flow dividers or flow distributors for ensuring a continuous equal distribution of a fluid to several different outlets are well known. Conventionally, a plurality of spaced sets of metering spur gears is provided and a common inlet conduit is connected to each of the gear sets. The output fluid from each of the gear sets should then always be balanced regardless of the hydraulic impedance in any of the outlet channels. Configurations of this general type are described, for example, in U.S. Pat. Nos. 2,291,578 to Johnson, 3,601,139 to Kontranowski, 3,495,610 to Van Aken, Jr., 2,343,912 to Lauck and 796,724 to Hewitt. Flow dividers of this general type, consisting of a series of small positive displacement gear pumps all driven in tandem by the pressure of fuel against the inlet side of the gears in each pump in the series, are presently employed in gas turbines for distributing liquid fuel between different combustion can fuel nozzles of the gas turbine.
The distribution of equal quantities of fuel to the different gas turbine fuel nozzles is particularly important under at least two operating conditions. The first is during turbine start-up when the flow divider must compensate for the differences in elevation between the various fuel nozzles. The second condition is during normal operation when, after hundreds of hours of operation, each of the fuel nozzles may accumulate different thicknesses of coke deposits. Nevertheless, the flow dividers are required to supply equal quantities of fuel to the various fuel nozzles, even though the back-pressures in the fuel lines to the individual fuel nozzles may vary from each other due to unequal coking of the various fuel nozzles.
Flow divider designs, which are now in use in gas turbines, generally use the liquid fuel oil being divided as the lubricating fluid for the bearings of the metering gear shafts. This lubrication is satisfactory when using clean fuels; however, when running with certain crude or residual fuels, which contain corrosion causing constituents, the metering shaft lubrication has been unsatisfactory, in that, excessive wear and failure of the bearings has occurred. The gas turbine flow dividers which are presently in use also require relatively complex assembly and disassembly when the device is to be inspected or maintained. Moreover, the designs which are presently used must be tailor-made for different shaft rotation speeds and do not lend themselves to manufacturing standardization.
Finally, the designs presently in use are not tolerant of thermal excursions in the fluid being metered. When such excursions occur, the various components of the flow divider undergo thermal expansion or contraction to a degree that is proportional to the temperature rise of the individual part and its coefficient of thermal expansion. Frequently when such expansion or contraction occurs, operating clearances are reduced, and binding or seizing of the flow divider parts may occur. In gas turbine operation, such binding or seizing of the flow divider can cause a loss of fuel to the combustion system, and the consequent shut down of the gas turbine. It should be noted that thermal transients are common in gas turbines which operate on crude or residual fuels, as usually these fuels have to be heated to temperatures as high as 250.degree. F. in order to achieve proper viscosity for combustion, and in order to ensure that all the wax particles in the fuel have been dissolved. However, the gas turbines start up and shut down on distillate fuel, which is usually at ambient temperature. It is during the changeover, from the distillate fuel to the crude or residual fuel, that a temperature transient up to about 200.degree. F. is experienced. This thermal transient takes place in a matter of a few seconds.
In order to overcome some of these deficiencies in the use of a flow divider for apportioning fuel to the turbine fuel nozzles, U.S. Pat. No. 4,328,824 issued to Kiernan, et al., describes a flow divider with metering gears, unrestrained spacers therebetween and lubricated end roller bearings. The patent describes a flow divider including a plurality of metering gear sets rigidly fixed on two parallel common continuous shafts. Each of the gear sets is separated from adjacent gear sets by separator spacers which are also fixed on the shafts, but free to move in a common radial cavity formed by a split housing enclosing the shafts and gears. Accordingly, the gears are "fixed" in position, which the separator spacers are free to slide in an axial direction. The two parallel common continuous shafts are supported at each end by roller bearings, which are separated from the gear sets by means of seals and which are lubricated by a separate source of lubricant. The roller bearings on one end of the parallel shafts are fixed in an axial location relative to the shafts and the roller bearings on the other end are free to move axially without experiencing binding or seizing of the shafts or roller bearings.
Under ideal conditions, with equal line impedances downstream of the flow divider, the Kiernan, et al. flow divider can provide equal distribution of flow to all nozzles. However, once an impedance develops in any one line the higher resulting back pressure creates an imbalanced axial force on the separator spacers associated with that line's respective gear sets. This axial force displaces the separator spacers further away from the gear sets, increasing the side clearances and, thus, decreasing the gear set efficiency. This results in an unsatisfactory condition.
In addition, experience has shown that although the split housing design of the Kiernan, et al. flow divider facilitates inspection of the device's internals, this feature alone does not aid a mechanic in establishing proper gear-tip to housing clearances. Quite obviously, once the top housing has been removed, there is no way of directly measuring these clearances. This would present a major problem for field repairs where sophisticated techniques for indirect measurement are limited or nonexistent.
It is therefore an object of the present invention to provide an improved flow divider such that metering efficiency is improved by rigidly fixing the separator spacers while allowing the gear sets to be free to slide.
It is another object of the present invention to provide an improved flow divider which includes a means for directly measuring critical gear set clearances while still retaining the split housing concept.